This invention relates to a reframe circuit which serves to maintain each of a plurality of digital groups of time division multiplexed channels in frame synchronization, that is, in an "in-frame" condition.
It is a commonplace in digital transmission to incorporate a marker pulse (i.e., framing bit) in a preassigned position in a digital data bit stream for the purpose of maintaining the receiving apparatus in a synchronous relationship to the transmitting apparatus. Such synchronization is essential for correct reconstitution of a message and, in the case of a time division multiplex system, for correct distribution of the several messages to their intended subscribers. To this end, a digital transmission system invariably includes frame detection circuitry for monitoring and determining the in-frame or out-of-frame condition of a received digital data bit stream; and, when the digital bit stream goes out-of-frame (i.e., loss of synchronization) vis-a-vis a locally generated framing pattern, a reframe circuit goes through a reframing operation to recapture frame synchronization. This is standard operating procedure in the digital transmission field.
In the past, pulse code modulation (PCM) digital data terminals have performed the task of frame synchronization, as well as signaling extraction, etc., on a per "digroup" basis - a digroup or digital group comprising a plurality of time division multiplexed PCM messages and multiplexed framing and signaling bits; see the article "The D3 Channel Bank" by W. B. Gaunt and J. B. Evans, Jr., Bell Laboratories Record, August 1972, pages 229-233, and the references cited therein. The per digroup partitioning of these functions has heretofore resulted in efficient terminal design.
With increasing digital traffic, it is not uncommon now to find proposals for multiplexing a plurality of digroups for transmission to a remote location over a common transmission facility or alternatively for multiplexing a plurality of received digroups on to a common bus at a switching center. These two cases are somewhat analogous and present the same problem with regard to reframing. Conventional practice would suggest carrying out the reframing operation on a per digroup basis using plural reframers to respectively maintain each of the plurality of multiplexed digroups in frame synchronization. The obvious disadvantage of this approach is, of course, its complexity and redundancy in reframe circuitry.
The U.S. Pat. No. 3,770,897 to R. H. Haussmann et al., issued Nov. 6, 1973, appears to suggest carrying out the frame synchronization operation for a plurality of multiplexed digital groups on a shared basis, but this proposal is really a hybrid of the per digroup approach noted above. The system of the patent functions as a sequential machine that monitors and reframes the multiplexed groups in a mutually exclusive fashion. That is, each digital group is separately dealt with over a number of frames to determine the in-frame or out-of-frame status of the same to achieve frame synchronization if a out-of-frame condition exists. But while a given group is being so handled, the other digital groups are ignored.